Thread: Fuses
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Old 17th Jan 2008, 12:26 am   #23
G Barham
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Join Date: Mar 2005
Posts: 46
Default Re: Fuses

As I mentioned earlier... don't forget the discrimination.

There are curves published for all types of fuses and MCB's. Basically the curve is a trace of current verses disconnection time and competent designers can look at the curves (normmally using both curves on one graph) and see if there are any overlaps.

If there is an overlap, it's possible for the higher rated fuse/MCB to blow before the lower rated one.

For simple house wiring and small industrial systems there are many computer progs. which do it all for you...you just type in the cable sizes, the known or measured impedances and the size/type of the chosen protection device and low and behold, the print out lets you know the current (ex. pun) situation.

For large systems and those getting close to supply transformers it's often a good idea to do a manual check.

A blown fuse in a plug top is one thing but a wrong setting on a 5000A ACB which could cause pieces of 100mm X 20mm copper bars to vaporise and blow a piece of sheet steel weighing 20Kg over 100 yds is another.

I think I'll try and stick with the mA... it makes sense !

Regards
Tony.
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